1. Field of the Invention
This invention relates to devices for providing a flow of gases at a pressure above atmospheric pressure to a user for therapeutic purposes.
This invention also relates to patient interfaces for use with devices which provide a gases stream at a pressure above atmospheric to a user for therapeutic purposes.
This invention also relates to methods of using patient interfaces of known geometry and dimensions to provide improved real time adjustment of the characteristics of a gases stream provided to a user for therapeutic purposes.
This invention also relates to methods of using patient interfaces of known geometry and dimensions to improve the initial or in-use characteristics of a gases stream provided to a user for therapeutic purposes.
2. Summary of the Prior Art
Devices or systems for providing a humidified gases flow to a patient for therapeutic purposes are well known in the art. Systems for providing therapy of this type, for example CPAP therapy, have a structure where gases at a pressure above atmospheric are delivered from a blower (also known as a compressor, an assisted breathing unit, a fan unit, a flow generator or a pressure generator) to a humidifier chamber downstream from the blower, where they are heated and humidified, and then provided to a user via a user interface. Examples of commonly used interfaces are:
1) a full face mask,
2) a nasal mask (covering the entire nose),
3) an oral mask sealing onto the mouth,
4) nasal pillows (a pair of conduits which provide a gases stream to a user through the users nostrils, contacting and sealing around the flares of the user),
5) a nasal cannula (a pair of narrow-bore conduits that pass into the nostrils of a user without sealing on the nares),
6) a combination of the above.
Other forms of interface can be used—for example tracheostomy fittings or similar. However, these are less common than those listed above.
Generally, the interface is connected to an outlet of the humidifier by a flexible conduit or similar. The interface is usually held in position on a user's head by a headgear. It is common to make at least part of the headgear from soft adjustable straps, for example Neoprene or similar.
Interfaces used with CPAP devices are provided with a leak path known as a bias flow which has the purpose of venting exhaled air to atmosphere. This prevents the patient re-breathing carbon dioxide contained in the exhaled breath. For example, the Flexifit 432 full face mask includes a number of small holes in the shell of the mask passing from the inside to the outside to act as bias vent holes.
CPAP therapy is intended to provide a fixed-pressure or constant pressure to a user. However, in reality, it is almost impossible to provide a fixed pressure for all flow levels. Variations on basic CPAP blowers have been introduced over time in order to address shortcomings.
For example, some blowers are adapted so as to provide VPAPυ or BiPAP® (Variable/Bilevel Positive Airway Pressure) where two levels of pressure are provided by the blower: one for inhalation (IPAP) and a lower pressure during exhalation (EPAP).
Ramping is possible with some blowers. This is a method used at the beginning of a users sleep cycle to allow the user to fall asleep more easily. The pressure provided by the blower at the start of a sleep cycle is lower than what is ideally required. The pressure provided by the blower is gradually increased to the prescribed level over a period of time, allowing the user to fall asleep before the full pressure is applied.
A blower that provides exhalation pressure relief is adapted so that there is a short drop in pressure during exhalation to reduce the effort required by a user to exhale. This feature is known by the trade name C-Flex® in some blowers manufactured by Respironics, and by the trade name EPR™ in the machine manufactured by ResMed.
Another disadvantage that some users can experience when using a mask as their interface is as follows: when the user exhales against an incoming pressurised stream of gases, this can cause leaks around the edges of the mask as the mask partially lifts away from, or is pushed off, the surface of a user's face. This can cause a user to wake up as gases wash across their face or jet into their eyes. It can also affect the efficiency of the treatment as not all the gas provided by the blower is reaching the user (some escapes before it reaches the user). In response to mask leak, it is common for a user or health professional to tighten the straps of the headgear. This can lead to the mask feeling uncomfortable for a user and with time can cause pressure sores and/or soft-tissue abrasions at the nasal bridge. The user can in response discontinue their therapy, or at least be unfavourably inclined towards continuing.
It should also be noted that uncontrolled mask leak is undesirable and a great deal of effort has been put into measuring and minimising mask leak, with the preferred intention of eliminating it entirely. One way of helping to avoid mask leak is to add a one-way or bias valve to the system, on or close to the mask or interface. This allows exhaled gases to be intentionally vented to atmosphere, and helps to avoid leak around the edges of a mask or other sealed interface. A mask vent is described in U.S. Pat. No. 6,662,803. A mask vent of different design is described in EP 1275412. Several other solutions have also been suggested. These can be used either alone or for example in tandem with mask vents or other solutions. U.S. Pat. No. 6,123,074 discloses a system where the mask includes a suitable exhaust port, and where pressure in the breathing system is constantly monitored and a pressure controller downstream of the flow generator (between the mask and the flow generator) acts to maintain a constant pressure within the conduit. U.S. Pat. No. 6,526,974 discloses a CPAP device where the size of the inlet to the blower or flow generator can be varied, or where the size of the inlet is automatically varied, in response to the needs of the user. An exhalation path is provided so that the back pressure in the system is limited.
If the mask or patient interface is provided with bias vent holes, there is always going to be some level of leakage through the vent holes. A typical level of exhaust flow through bias vent holes in a mask is in the region of 35 Liters per minute, at a pressure level of 10 cmH.sub.2O above atmospheric pressure. This is a known level of leak and can be compensated for by adjusting the settings of the blower and the humidifier. However, a portion of the heated, humidified gases is lost (sacrificed) before reaching the user.
In contrast, one advantage of using a nasal cannula is that all the flow is provided to the nostrils and does not vent (through bias vent holes) before reaching the nasal cavity. However, unsealed nasal cannulas have their own shortcomings. For example, the size of the space between the nostrils and the cannula is unknown. Therefore, the amount of leak from around the cannula is unknown and uncontrollable, even when the pressure at the cannula is known, and the pressure delivered to the nares is unknown or uncontrollable even if the flow delivered to the cannula is known. Also, when using a cannula, it is impossible to avoid the entrainment of dry, unheated atmospheric air into the nostrils of a user.
Another problem encountered with unsealed high flow nasal cannula is the noise that is generated by air flowing between the nares and the outside of the nasal cannula. This can be especially problematic when a nearly constant flow is provided to the user. In this situation the flow exiting to atmosphere varies significantly as the patient exhales and inhales creating noise that varies in intensity over the breath cycle. The amount of noise is strongly dependant on the position and orientation of the cannula in nares as this affects the velocity of the exhaust (leaked) gas.
High flow nasal cannula are used to provide gas for treatment of illnesses including COPD, CF and OSA amongst many others. It is sometimes desirable to set a flow rate through the cannula that is sufficient to meet the patient's maximum inspiratory demand, but which does not significantly exceed that required to meet maximum inspiratory demand. This can be difficult to achieve reliably due to the unknown resistance to flow of air passing between outside of the cannula and the inside of the nares.
For treatment of some conditions the pressure at the nares is important. For example, in the treatment of some respiratory conditions, such as COPD, application of pressure during expiration can be beneficial. With a non-sealing nasal cannula the amount of pressure delivered is unknown as explained previously.
Nasal pillows are a variation on the basic cannula structure which are intended to go some way toward solving the problems which are experienced with nasal cannulas. In nasal pillows, the narrow elongated inlet portions of the cannula are replaced with soft flexible portions which generally conform to the geometry of a users nostrils, and which flex to seal against the nares of a user in use. This helps to avoid entrainment of atmospheric air into the nostrils of a user when inhaling. However, it can be difficult to exhale against the stream of air provided when using nasal pillows, and uncontrolled leaks can occur. It can also be difficult to create an adequate seal against the nares if the users nasal geometry is significantly different from that of the pillows.
Further disadvantages arise when attempting to measure the flow and the pressure at various points in the system. It can be difficult to relate actual measured data to the breathing cycle of a user in order to monitor or adjust (either manually or automatically through a feedback mechanism in the blower control circuitry) controllable system parameters such as the fan speed (to alter flow and pressure) or the energy provided to the heater mechanism of the humidifier (to alter the temperature and humidity of the gases) in order to provide the most effective therapy. It is especially difficult to accurately assess pressure and flow e.g. in the upper airway of a user when leaks occur between the face or nose of the patient and the interface.
Other forms of treatment deliver a high flow of gas (dry or humidified) through an unsealed interface such as a nasal cannula. In these applications there is a large leak out of the nostrils around the nasal cannula. The large leak means that the pressure in the airway is relatively low compared to CPAP treatment.
A disadvantage of the high flow nasal cannula system is that it is especially difficult to assess the values of pressure and actual patient flow during a patient's breath because the resistance to flow for gas passing between the cannulae and the nares can be variable depending on the position of the cannula and shape and size of the nare.
Obstructions in some patients' airways during sleep can cause limited airflow, leading to apnoea, hypopnoea, or snoring. The obstruction is often a collapsed pharynx. The obstruction may be a partial airway obstruction, leading to altered characteristics of the airflow. A hypopnoea is a reduction of flow that is greater than 30%, but less than 90% of baseline, for a period of at least 10 seconds and which is accompanied by oxygen desaturation greater than 4% from baseline. An apnoea is similar but airflow is reduced by greater than 90% from baseline. Each of these conditions frequently leads to sleep deprivation.
It is well known to treat patients suffering from sleep deprivation with positive airway pressure therapy (“PAP”). This therapy can be Continuous Positive Airway Pressure (“CPAP”), Variable Positive Airway Pressure (“VPAP”), Bi-level Positive Airway Pressure (“BiPAP”), or any of numerous other forms of respiratory therapy. The application of positive pressure to the patient's pharynx helps minimize or prevent this collapse. Positive airway pressure therapy is currently applied by means of an apparatus containing a pressure source, typically a blower, through a tube to a mask, which the patient wears in bed.
It is desired to control the applied pressure. Too little pressure tends not to solve the problem. Too much pressure tends to cause discomfort to the patient, such as drying out of the mouth and pharynx, as well as difficulty in exhaling against the applied pressure. The difficulty in applying optimum pressure is that incidents of airway obstruction come and go through the course of a night's sleep. One solution is to try to find an optimum pressure for a particular patient and maintain that pressure. This method requires the patient's stay at a sleep clinic, where sleep specialists can monitor the patient's course of breathing throughout one or more night's sleep, prescribe the appropriate pressure for that patient, and then set the apparatus to deliver the appropriate pressure. This method is, of course, inconvenient as well as expensive to the patient and tends to be inaccurate, as a typical patient will not sleep the same when away from familiar bedding and surroundings.
Accordingly, it is desirable to be able to adjust the applied pressure without requiring the patient to attend at a sleep centre. Devices which are aimed at adjusting the applied pressure automatically are generally known as ‘Auto adjusting Devices’. Various methods of in-home adjustments have been considered. One method generally thought to be effective is to monitor the patient to try to anticipate the onset of an obstructed airway, and to adjust the pressure in response. When an elevated upper airway resistance or flow obstruction is anticipated or underway, the apparatus increases the applied pressure. When the patient returns to normal sleep, the applied pressure is reduced. The problem then, is to determine when a flow obstruction is occurring or is about to occur. It is desired to anticipate correctly in order to avoid the problems set forth above for when too much or too little pressure is applied.
Various methods have been proposed to solve this problem. In U.S. Pat. No. 5,107,831 to Halpern, an apparatus monitors the airflow to the patient and posits an event of airway obstruction when the patient's breath fails to meet a predetermined threshold of flow rate or duration. In U.S. Pat. No. 5,134,995 to Gruenke, an apparatus monitors the airflow to the patient and analyzes the shape of the flow versus time waveform. If the shape of this waveform tends to be flattened, that is, more similar to a plateau than to a sinusoid, the apparatus posits an event of airway obstruction. In U.S. Pat. No. 5,245,995 to Sullivan, an apparatus monitors the patient's sound with a microphone. If audible snores are detected, the apparatus posits an event of airway obstruction. Similarly, in U.S. Pat. No. 5,953,713 to Behbehani, an apparatus measures the total pressure within an interface placed over a patient's airway and inputs frequency data in the range 100 to 150 Hz into a neural network to determine the presence of a pharyngeal wall vibration (a snore) which, according to Behbehani, is a precursor to sleep disorder breathing.
An alternative method described in patent US20060027234A1: Auto-titrating method and apparatus. This specification discusses obtaining information from the frequency range of zero to 25 HZ in the frequency domain of the flow, and adjusting the pressure based on the information obtained.
Prior art methods have not proven totally satisfactory in controlling the applied pressure during PAP therapy. For example, the '713 patent, by measuring in the range of 100 to 150 Hz, essentially tests for snoring and does not measure or analyze any information concerning partial airway obstruction, as this information is found in the lower frequency range 0 to 25 Hz.
It is an object of the present invention to provide a breathing assistance apparatus which goes some way to overcoming the abovementioned disadvantages or which at least provides the public or industry with a useful choice.